In recent years, many imaging devices such as surveillance cameras and camcorders are equipped with an auto focus function to automatically adjust focus. Hereinafter, the auto focus is described as “AF” in drawings.
In the camera equipped with the auto focus function, a focus lens is automatically controlled to an in-focus position of a subject when the subject is out of focus due to a change in a zoom lens position or movement of the subject.
The camera equipped with the auto focus function generally causes micro vibration of the focus lens in an optical axis direction near the in-focus position (hereinafter, referred to as wobbling operation) to thereby detect the direction for focusing according to a change in an auto focus evaluation value and then move the focus lens in this direction.
In a rough classification, the auto focus function includes an active system and a passive system. The active system is a system in which distance to the subject is detected based on time until a reflected wave of e.g. infrared or ultrasound, which is delivered to the subject, returns and irradiation angle to the subject. The passive system is a system in which the distance to a subject is detected based on an optical image obtained by light reception by a lens.
Examples of the passive system include a phase difference detection system and a contrast detection system. The phase difference detection system is a system in which the optical image obtained by light reception is separated by a separator lens and a defocus amount (amount of separation from the in-focus position) calculated from the phase difference thereof is employed as a control value of an auto focus control to minimize defocus amount.
The contrast detection system is a system in which a contrast signal of a picture signal obtained by imaging by an imaging element is detected and employed as an evaluation value of the auto focus control. The evaluation value is controlled to maximize the contrast signal. This contrast detection system requires no dedicated component for the auto focus control, and therefore is widely employed generally in the cameras such as surveillance cameras and camcorders.
The contrast detection system utilizes characteristics that the subject picture comes into focus at a higher degree when high-frequency components of the picture signal existing at e.g. a contour part when the subject is imaged become larger. This is equivalent to that luminance level difference between a bright part and a dark part of a contour part of the subject, i.e. the contrast, becomes larger. Therefore, to focus a desired subject, the focus lens is moved in such a direction that the high-frequency components of the picture signal become larger.
The contrast detection system is referred to also as a hill climbing system. When the focus lens is moved and the contrast signal is obtained from a picture signal arising from imaging at each focus lens position, the peak (hill) of the contrast signal is a correct focus position. In the contrast detection system, this contrast signal is employed as the auto focus evaluation value and the focus lens is moved as if climbing the peak (hill). Therefore, this system is referred to as the hill climbing system.
In patent document 1, an invention is described in which focusing accuracy is enhanced by allowing focus lens speed to be variable depending on a ratio of a focal point evaluation value in a predetermined time and change amount of lens position.
In patent document 2, an invention is described in which the auto focus evaluation value is corrected based on a high-luminance count value (ratio of high-luminance image) and the in-focus point is identified based on the corrected value.
In patent document 3, an invention is described in which two filters having different cut-off frequency characteristics are used and focus lens movement speed (or movement amount) is set depending on the ratio of two focal point evaluation values to enhance the focusing accuracy.
In patent document 4, an invention is described in which the wobbling amplitude is increased to enhance the focusing accuracy when a focal point evaluation value at start of wobbling operation is determined small and a comparative angle change is also small.